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This article explains the new features in Python 2.7. Python 2.7 was released
on July 3, 2010.

Numeric handling has been improved in many ways, for both
floating-point numbers and for the Decimal class.
There are some useful additions to the standard library, such as a
greatly enhanced unittest module, the argparse module
for parsing command-line options, convenient OrderedDict
and Counter classes in the collections module,
and many other improvements.

Python 2.7 is planned to be the last of the 2.x releases, so we worked
on making it a good release for the long term. To help with porting
to Python 3, several new features from the Python 3.x series have been
included in 2.7.

This article doesn’t attempt to provide a complete specification of
the new features, but instead provides a convenient overview. For
full details, you should refer to the documentation for Python 2.7 at
https://docs.python.org. If you want to understand the rationale for
the design and implementation, refer to the PEP for a particular new
feature or the issue on https://bugs.python.org in which a change was
discussed. Whenever possible, “What’s New in Python” links to the
bug/patch item for each change.

Python 2.7 is the last major release in the 2.x series, as the Python
maintainers have shifted the focus of their new feature development efforts
to the Python 3.x series. This means that while Python 2 continues to
receive bug fixes, and to be updated to build correctly on new hardware and
versions of supported operated systems, there will be no new full feature
releases for the language or standard library.

However, while there is a large common subset between Python 2.7 and Python
3, and many of the changes involved in migrating to that common subset, or
directly to Python 3, can be safely automated, some other changes (notably
those associated with Unicode handling) may require careful consideration,
and preferably robust automated regression test suites, to migrate
effectively.

This means that Python 2.7 will remain in place for a long time, providing a
stable and supported base platform for production systems that have not yet
been ported to Python 3. The full expected lifecycle of the Python 2.7
series is detailed in PEP 373.

Some key consequences of the long-term significance of 2.7 are:

As noted above, the 2.7 release has a much longer period of maintenance
when compared to earlier 2.x versions. Python 2.7 is currently expected to
remain supported by the core development team (receiving security updates
and other bug fixes) until at least 2020 (10 years after its initial
release, compared to the more typical support period of 18–24 months).

As the Python 2.7 standard library ages, making effective use of the
Python Package Index (either directly or via a redistributor) becomes
more important for Python 2 users. In addition to a wide variety of third
party packages for various tasks, the available packages include backports
of new modules and features from the Python 3 standard library that are
compatible with Python 2, as well as various tools and libraries that can
make it easier to migrate to Python 3. The Python Packaging User Guide provides guidance on downloading and
installing software from the Python Package Index.

While the preferred approach to enhancing Python 2 is now the publication
of new packages on the Python Package Index, this approach doesn’t
necessarily work in all cases, especially those related to network
security. In exceptional cases that cannot be handled adequately by
publishing new or updated packages on PyPI, the Python Enhancement
Proposal process may be used to make the case for adding new features
directly to the Python 2 standard library. Any such additions, and the
maintenance releases where they were added, will be noted in the
New Features Added to Python 2.7 Maintenance Releases section below.

For projects wishing to migrate from Python 2 to Python 3, or for library
and framework developers wishing to support users on both Python 2 and
Python 3, there are a variety of tools and guides available to help decide
on a suitable approach and manage some of the technical details involved.
The recommended starting point is the Porting Python 2 Code to Python 3 HOWTO guide.

For Python 2.7, a policy decision was made to silence warnings only of
interest to developers by default. DeprecationWarning and its
descendants are now ignored unless otherwise requested, preventing
users from seeing warnings triggered by an application. This change
was also made in the branch that became Python 3.2. (Discussed
on stdlib-sig and carried out in issue 7319.)

In previous releases, DeprecationWarning messages were
enabled by default, providing Python developers with a clear
indication of where their code may break in a future major version
of Python.

However, there are increasingly many users of Python-based
applications who are not directly involved in the development of
those applications. DeprecationWarning messages are
irrelevant to such users, making them worry about an application
that’s actually working correctly and burdening application developers
with responding to these concerns.

You can re-enable display of DeprecationWarning messages by
running Python with the -Wdefault (short form:
-Wd) switch, or by setting the PYTHONWARNINGS
environment variable to "default" (or "d") before running
Python. Python code can also re-enable them
by calling warnings.simplefilter('default').

The repr() of a float x is shorter in many cases: it’s now
based on the shortest decimal string that’s guaranteed to round back
to x. As in previous versions of Python, it’s guaranteed that
float(repr(x)) recovers x.

Float-to-string and string-to-float conversions are correctly rounded.
The round() function is also now correctly rounded.

Regular Python dictionaries iterate over key/value pairs in arbitrary order.
Over the years, a number of authors have written alternative implementations
that remember the order that the keys were originally inserted. Based on
the experiences from those implementations, 2.7 introduces a new
OrderedDict class in the collections module.

The OrderedDict API provides the same interface as regular
dictionaries but iterates over keys and values in a guaranteed order
depending on when a key was first inserted:

Comparing an OrderedDict with a regular dictionary
ignores the insertion order and just compares the keys and values.

How does the OrderedDict work? It maintains a
doubly-linked list of keys, appending new keys to the list as they’re inserted.
A secondary dictionary maps keys to their corresponding list node, so
deletion doesn’t have to traverse the entire linked list and therefore
remains O(1).

The standard library now supports use of ordered dictionaries in several
modules.

The ConfigParser module uses them by default, meaning that
configuration files can now be read, modified, and then written back
in their original order.

The json module’s JSONDecoder class
constructor was extended with an object_pairs_hook parameter to
allow OrderedDict instances to be built by the decoder.
Support was also added for third-party tools like
PyYAML.

To make program output more readable, it can be useful to add
separators to large numbers, rendering them as
18,446,744,073,709,551,616 instead of 18446744073709551616.

The fully general solution for doing this is the locale module,
which can use different separators (”,” in North America, ”.” in
Europe) and different grouping sizes, but locale is complicated
to use and unsuitable for multi-threaded applications where different
threads are producing output for different locales.

Therefore, a simple comma-grouping mechanism has been added to the
mini-language used by the str.format() method. When
formatting a floating-point number, simply include a comma between the
width and the precision:

This mechanism is not adaptable at all; commas are always used as the
separator and the grouping is always into three-digit groups. The
comma-formatting mechanism isn’t as general as the locale
module, but it’s easier to use.

The argparse module for parsing command-line arguments was
added as a more powerful replacement for the
optparse module.

This means Python now supports three different modules for parsing
command-line arguments: getopt, optparse, and
argparse. The getopt module closely resembles the C
library’s getopt() function, so it remains useful if you’re writing a
Python prototype that will eventually be rewritten in C.
optparse becomes redundant, but there are no plans to remove it
because there are many scripts still using it, and there’s no
automated way to update these scripts. (Making the argparse
API consistent with optparse‘s interface was discussed but
rejected as too messy and difficult.)

In short, if you’re writing a new script and don’t need to worry
about compatibility with earlier versions of Python, use
argparse instead of optparse.

argparse has much fancier validation than optparse; you
can specify an exact number of arguments as an integer, 0 or more
arguments by passing '*', 1 or more by passing '+', or an
optional argument with '?'. A top-level parser can contain
sub-parsers to define subcommands that have different sets of
switches, as in svncommit, svncheckout, etc. You can
specify an argument’s type as FileType, which will
automatically open files for you and understands that '-' means
standard input or output.

The logging module is very flexible; applications can define
a tree of logging subsystems, and each logger in this tree can filter
out certain messages, format them differently, and direct messages to
a varying number of handlers.

All this flexibility can require a lot of configuration. You can
write Python statements to create objects and set their properties,
but a complex set-up requires verbose but boring code.
logging also supports a fileConfig()
function that parses a file, but the file format doesn’t support
configuring filters, and it’s messier to generate programmatically.

Python 2.7 adds a dictConfig() function that
uses a dictionary to configure logging. There are many ways to
produce a dictionary from different sources: construct one with code;
parse a file containing JSON; or use a YAML parsing library if one is
installed. For more information see Configuration functions.

The following example configures two loggers, the root logger and a
logger named “network”. Messages sent to the root logger will be
sent to the system log using the syslog protocol, and messages
to the “network” logger will be written to a network.log file
that will be rotated once the log reaches 1MB.

importloggingimportlogging.configconfigdict={'version':1,# Configuration schema in use; must be 1 for now'formatters':{'standard':{'format':('%(asctime)s%(name)-15s ''%(levelname)-8s%(message)s')}},'handlers':{'netlog':{'backupCount':10,'class':'logging.handlers.RotatingFileHandler','filename':'/logs/network.log','formatter':'standard','level':'INFO','maxBytes':1000000},'syslog':{'class':'logging.handlers.SysLogHandler','formatter':'standard','level':'ERROR'}},# Specify all the subordinate loggers'loggers':{'network':{'handlers':['netlog']}},# Specify properties of the root logger'root':{'handlers':['syslog']},}# Set up configurationlogging.config.dictConfig(configdict)# As an example, log two error messageslogger=logging.getLogger('/')logger.error('Database not found')netlogger=logging.getLogger('network')netlogger.error('Connection failed')

Three smaller enhancements to the logging module, all
implemented by Vinay Sajip, are:

Logger instances gained a getChild()
method that retrieves a descendant logger using a relative path.
For example, once you retrieve a logger by doing log=getLogger('app'),
calling log.getChild('network.listen') is equivalent to
getLogger('app.network.listen').

The LoggerAdapter class gained an
isEnabledFor() method that takes a
level and returns whether the underlying logger would
process a message of that level of importance.

The dictionary methods keys(), values(), and
items() are different in Python 3.x. They return an object
called a view instead of a fully materialized list.

It’s not possible to change the return values of keys(),
values(), and items() in Python 2.7 because
too much code would break. Instead the 3.x versions were added
under the new names viewkeys(), viewvalues(),
and viewitems().

The syntax for set literals has been backported from Python 3.x.
Curly brackets are used to surround the contents of the resulting
mutable set; set literals are
distinguished from dictionaries by not containing colons and values.
{} continues to represent an empty dictionary; use
set() for an empty set.

Conversions between floating-point numbers and strings are
now correctly rounded on most platforms. These conversions occur
in many different places: str() on
floats and complex numbers; the float and complex
constructors;
numeric formatting; serializing and
deserializing floats and complex numbers using the
marshal, pickle
and json modules;
parsing of float and imaginary literals in Python code;
and Decimal-to-float conversion.

Related to this, the repr() of a floating-point number x
now returns a result based on the shortest decimal string that’s
guaranteed to round back to x under correct rounding (with
round-half-to-even rounding mode). Previously it gave a string
based on rounding x to 17 decimal digits.

The rounding library responsible for this improvement works on
Windows and on Unix platforms using the gcc, icc, or suncc
compilers. There may be a small number of platforms where correct
operation of this code cannot be guaranteed, so the code is not
used on such systems. You can find out which code is being used
by checking sys.float_repr_style, which will be short
if the new code is in use and legacy if it isn’t.

Conversions from long integers and regular integers to floating
point now round differently, returning the floating-point number
closest to the number. This doesn’t matter for small integers that
can be converted exactly, but for large numbers that will
unavoidably lose precision, Python 2.7 now approximates more
closely. For example, Python 2.6 computed the following:

Integer division is also more accurate in its rounding behaviours. (Also
implemented by Mark Dickinson; issue 1811.)

Implicit coercion for complex numbers has been removed; the interpreter
will no longer ever attempt to call a __coerce__() method on complex
objects. (Removed by Meador Inge and Mark Dickinson; issue 5211.)

The str.format() method now supports automatic numbering of the replacement
fields. This makes using str.format() more closely resemble using
%s formatting:

The auto-numbering takes the fields from left to right, so the first {...}
specifier will use the first argument to str.format(), the next
specifier will use the next argument, and so on. You can’t mix auto-numbering
and explicit numbering – either number all of your specifier fields or none
of them – but you can mix auto-numbering and named fields, as in the second
example above. (Contributed by Eric Smith; issue 5237.)

Complex numbers now correctly support usage with format(),
and default to being right-aligned.
Specifying a precision or comma-separation applies to both the real
and imaginary parts of the number, but a specified field width and
alignment is applied to the whole of the resulting 1.5+3j
output. (Contributed by Eric Smith; issue 1588 and issue 7988.)

The ‘F’ format code now always formats its output using uppercase characters,
so it will now produce ‘INF’ and ‘NAN’.
(Contributed by Eric Smith; issue 3382.)

A low-level change: the object.__format__() method now triggers
a PendingDeprecationWarning if it’s passed a format string,
because the __format__() method for object converts
the object to a string representation and formats that. Previously
the method silently applied the format string to the string
representation, but that could hide mistakes in Python code. If
you’re supplying formatting information such as an alignment or
precision, presumably you’re expecting the formatting to be applied
in some object-specific way. (Fixed by Eric Smith; issue 7994.)

The int() and long() types gained a bit_length
method that returns the number of bits necessary to represent
its argument in binary:

The import statement will no longer try an absolute import
if a relative import (e.g. from.osimportsep) fails. This
fixes a bug, but could possibly break certain import
statements that were only working by accident. (Fixed by Meador Inge;
issue 7902.)

It’s now possible for a subclass of the built-in unicode type
to override the __unicode__() method. (Implemented by
Victor Stinner; issue 1583863.)

When using @classmethod and @staticmethod to wrap
methods as class or static methods, the wrapper object now
exposes the wrapped function as their __func__ attribute.
(Contributed by Amaury Forgeot d’Arc, after a suggestion by
George Sakkis; issue 5982.)

When a restricted set of attributes were set using __slots__,
deleting an unset attribute would not raise AttributeError
as you would expect. Fixed by Benjamin Peterson; issue 7604.)

Two new encodings are now supported: “cp720”, used primarily for
Arabic text; and “cp858”, a variant of CP 850 that adds the euro
symbol. (CP720 contributed by Alexander Belchenko and Amaury
Forgeot d’Arc in issue 1616979; CP858 contributed by Tim Hatch in
issue 8016.)

The file object will now set the filename attribute
on the IOError exception when trying to open a directory
on POSIX platforms (noted by Jan Kaliszewski; issue 4764), and
now explicitly checks for and forbids writing to read-only file objects
instead of trusting the C library to catch and report the error
(fixed by Stefan Krah; issue 5677).

The Python tokenizer now translates line endings itself, so the
compile() built-in function now accepts code using any
line-ending convention. Additionally, it no longer requires that the
code end in a newline.

Extra parentheses in function definitions are illegal in Python 3.x,
meaning that you get a syntax error from deff((x)):pass. In
Python3-warning mode, Python 2.7 will now warn about this odd usage.
(Noted by James Lingard; issue 7362.)

A new environment variable, PYTHONWARNINGS,
allows controlling warnings. It should be set to a string
containing warning settings, equivalent to those
used with the -W switch, separated by commas.
(Contributed by Brian Curtin; issue 7301.)

For example, the following setting will print warnings every time
they occur, but turn warnings from the Cookie module into an
error. (The exact syntax for setting an environment variable varies
across operating systems and shells.)

A new opcode was added to perform the initial setup for
with statements, looking up the __enter__() and
__exit__() methods. (Contributed by Benjamin Peterson.)

The garbage collector now performs better for one common usage
pattern: when many objects are being allocated without deallocating
any of them. This would previously take quadratic
time for garbage collection, but now the number of full garbage collections
is reduced as the number of objects on the heap grows.
The new logic only performs a full garbage collection pass when
the middle generation has been collected 10 times and when the
number of survivor objects from the middle generation exceeds 10% of
the number of objects in the oldest generation. (Suggested by Martin
von Löwis and implemented by Antoine Pitrou; issue 4074.)

The garbage collector tries to avoid tracking simple containers
which can’t be part of a cycle. In Python 2.7, this is now true for
tuples and dicts containing atomic types (such as ints, strings,
etc.). Transitively, a dict containing tuples of atomic types won’t
be tracked either. This helps reduce the cost of each
garbage collection by decreasing the number of objects to be
considered and traversed by the collector.
(Contributed by Antoine Pitrou; issue 4688.)

Long integers are now stored internally either in base 2**15 or in base
2**30, the base being determined at build time. Previously, they
were always stored in base 2**15. Using base 2**30 gives
significant performance improvements on 64-bit machines, but
benchmark results on 32-bit machines have been mixed. Therefore,
the default is to use base 2**30 on 64-bit machines and base 2**15
on 32-bit machines; on Unix, there’s a new configure option
--enable-big-digits that can be used to override this default.

Apart from the performance improvements this change should be
invisible to end users, with one exception: for testing and
debugging purposes there’s a new structseq sys.long_info that
provides information about the internal format, giving the number of
bits per digit and the size in bytes of the C type used to store
each digit:

Another set of changes made long objects a few bytes smaller: 2 bytes
smaller on 32-bit systems and 6 bytes on 64-bit.
(Contributed by Mark Dickinson; issue 5260.)

The division algorithm for long integers has been made faster
by tightening the inner loop, doing shifts instead of multiplications,
and fixing an unnecessary extra iteration.
Various benchmarks show speedups of between 50% and 150% for long
integer divisions and modulo operations.
(Contributed by Mark Dickinson; issue 5512.)
Bitwise operations are also significantly faster (initial patch by
Gregory Smith; issue 1087418).

The implementation of % checks for the left-side operand being
a Python string and special-cases it; this results in a 1–3%
performance increase for applications that frequently use %
with strings, such as templating libraries.
(Implemented by Collin Winter; issue 5176.)

List comprehensions with an if condition are compiled into
faster bytecode. (Patch by Antoine Pitrou, back-ported to 2.7
by Jeffrey Yasskin; issue 4715.)

Converting an integer or long integer to a decimal string was made
faster by special-casing base 10 instead of using a generalized
conversion function that supports arbitrary bases.
(Patch by Gawain Bolton; issue 6713.)

The split(), replace(), rindex(),
rpartition(), and rsplit() methods of string-like types
(strings, Unicode strings, and bytearray objects) now use a
fast reverse-search algorithm instead of a character-by-character
scan. This is sometimes faster by a factor of 10. (Added by
Florent Xicluna; issue 7462 and issue 7622.)

The pickle and cPickle modules now automatically
intern the strings used for attribute names, reducing memory usage
of the objects resulting from unpickling. (Contributed by Jake
McGuire; issue 5084.)

The cPickle module now special-cases dictionaries,
nearly halving the time required to pickle them.
(Contributed by Collin Winter; issue 5670.)

As in every release, Python’s standard library received a number of
enhancements and bug fixes. Here’s a partial list of the most notable
changes, sorted alphabetically by module name. Consult the
Misc/NEWS file in the source tree for a more complete list of
changes, or look through the Subversion logs for all the details.

The bdb module’s base debugging class Bdb
gained a feature for skipping modules. The constructor
now takes an iterable containing glob-style patterns such as
django.*; the debugger will not step into stack frames
from a module that matches one of these patterns.
(Contributed by Maru Newby after a suggestion by
Senthil Kumaran; issue 5142.)

The binascii module now supports the buffer API, so it can be
used with memoryview instances and other similar buffer objects.
(Backported from 3.x by Florent Xicluna; issue 7703.)

There are three additional Counter methods.
most_common() returns the N most common
elements and their counts. elements()
returns an iterator over the contained elements, repeating each
element as many times as its count.
subtract() takes an iterable and
subtracts one for each element instead of adding; if the argument is
a dictionary or another Counter, the counts are
subtracted.

New method: The deque data type now has a
count() method that returns the number of
contained elements equal to the supplied argument x, and a
reverse() method that reverses the elements
of the deque in-place. deque also exposes its maximum
length as the read-only maxlen attribute.
(Both features added by Raymond Hettinger.)

The namedtuple class now has an optional rename parameter.
If rename is true, field names that are invalid because they’ve
been repeated or aren’t legal Python identifiers will be
renamed to legal names that are derived from the field’s
position within the list of fields:

Deprecated function: contextlib.nested(), which allows
handling more than one context manager with a single with
statement, has been deprecated, because the with statement
now supports multiple context managers.

The cookielib module now ignores cookies that have an invalid
version field, one that doesn’t contain an integer value. (Fixed by
John J. Lee; issue 3924.)

The ctypes module now always converts None to a C NULL
pointer for arguments declared as pointers. (Changed by Thomas
Heller; issue 4606.) The underlying libffi library has been updated to version
3.0.9, containing various fixes for different platforms. (Updated
by Matthias Klose; issue 8142.)

New method: the Decimal class gained a
from_float() class method that performs an exact
conversion of a floating-point number to a Decimal.
This exact conversion strives for the
closest decimal approximation to the floating-point representation’s value;
the resulting decimal value will therefore still include the inaccuracy,
if any.
For example, Decimal.from_float(0.1) returns
Decimal('0.1000000000000000055511151231257827021181583404541015625').
(Implemented by Raymond Hettinger; issue 4796.)

Comparing instances of Decimal with floating-point
numbers now produces sensible results based on the numeric values
of the operands. Previously such comparisons would fall back to
Python’s default rules for comparing objects, which produced arbitrary
results based on their type. Note that you still cannot combine
Decimal and floating-point in other operations such as addition,
since you should be explicitly choosing how to convert between float and
Decimal. (Fixed by Mark Dickinson; issue 2531.)

When using Decimal instances with a string’s
format() method, the default alignment was previously
left-alignment. This has been changed to right-alignment, which is
more sensible for numeric types. (Changed by Mark Dickinson; issue 6857.)

The difflib module now produces output that is more
compatible with modern diff/patch tools
through one small change, using a tab character instead of spaces as
a separator in the header giving the filename. (Fixed by Anatoly
Techtonik; issue 7585.)

The Distutils sdist command now always regenerates the
MANIFEST file, since even if the MANIFEST.in or
setup.py files haven’t been modified, the user might have
created some new files that should be included.
(Fixed by Tarek Ziadé; issue 8688.)

The doctest module’s IGNORE_EXCEPTION_DETAIL flag
will now ignore the name of the module containing the exception
being tested. (Patch by Lennart Regebro; issue 7490.)

The email module’s Message class will
now accept a Unicode-valued payload, automatically converting the
payload to the encoding specified by output_charset.
(Added by R. David Murray; issue 1368247.)

The Fraction class now accepts a single float or
Decimal instance, or two rational numbers, as
arguments to its constructor. (Implemented by Mark Dickinson;
rationals added in issue 5812, and float/decimal in
issue 8294.)

Ordering comparisons (<, <=, >, >=) between
fractions and complex numbers now raise a TypeError.
This fixes an oversight, making the Fraction
match the other numeric types.

New class: FTP_TLS in
the ftplib module provides secure FTP
connections using TLS encapsulation of authentication as well as
subsequent control and data transfers.
(Contributed by Giampaolo Rodola; issue 2054.)

The storbinary() method for binary uploads can now restart
uploads thanks to an added rest parameter (patch by Pablo Mouzo;
issue 6845.)

New function: cmp_to_key() will take an old-style comparison
function that expects two arguments and return a new callable that
can be used as the key parameter to functions such as
sorted(), min() and max(), etc. The primary
intended use is to help with making code compatible with Python 3.x.
(Added by Raymond Hettinger.)

New function: the gc module’s is_tracked() returns
true if a given instance is tracked by the garbage collector, false
otherwise. (Contributed by Antoine Pitrou; issue 4688.)

The gzip module’s GzipFile now supports the context
management protocol, so you can write withgzip.GzipFile(...)asf:
(contributed by Hagen Fürstenau; issue 3860), and it now implements
the io.BufferedIOBase ABC, so you can wrap it with
io.BufferedReader for faster processing
(contributed by Nir Aides; issue 7471).
It’s also now possible to override the modification time
recorded in a gzipped file by providing an optional timestamp to
the constructor. (Contributed by Jacques Frechet; issue 4272.)

Files in gzip format can be padded with trailing zero bytes; the
gzip module will now consume these trailing bytes. (Fixed by
Tadek Pietraszek and Brian Curtin; issue 2846.)

New attribute: the hashlib module now has an algorithms
attribute containing a tuple naming the supported algorithms.
In Python 2.7, hashlib.algorithms contains
('md5','sha1','sha224','sha256','sha384','sha512').
(Contributed by Carl Chenet; issue 7418.)

The default HTTPResponse class used by the httplib module now
supports buffering, resulting in much faster reading of HTTP responses.
(Contributed by Kristján Valur Jónsson; issue 4879.)

The HTTPConnection and HTTPSConnection classes
now support a source_address parameter, a (host,port) 2-tuple
giving the source address that will be used for the connection.
(Contributed by Eldon Ziegler; issue 3972.)

The ihooks module now supports relative imports. Note that
ihooks is an older module for customizing imports,
superseded by the imputil module added in Python 2.0.
(Relative import support added by Neil Schemenauer.)

New function: the inspect module’s getcallargs()
takes a callable and its positional and keyword arguments,
and figures out which of the callable’s parameters will receive each argument,
returning a dictionary mapping argument names to their values. For example:

Updated module: The io library has been upgraded to the version shipped with
Python 3.1. For 3.1, the I/O library was entirely rewritten in C
and is 2 to 20 times faster depending on the task being performed. The
original Python version was renamed to the _pyio module.

One minor resulting change: the io.TextIOBase class now
has an errors attribute giving the error setting
used for encoding and decoding errors (one of 'strict', 'replace',
'ignore').

The io.FileIO class now raises an OSError when passed
an invalid file descriptor. (Implemented by Benjamin Peterson;
issue 4991.) The truncate() method now preserves the
file position; previously it would change the file position to the
end of the new file. (Fixed by Pascal Chambon; issue 6939.)

New function: itertools.compress(data,selectors) takes two
iterators. Elements of data are returned if the corresponding
value in selectors is true:

itertools.compress('ABCDEF',[1,0,1,0,1,1])=>A,C,E,F

New function: itertools.combinations_with_replacement(iter,r)
returns all the possible r-length combinations of elements from the
iterable iter. Unlike combinations(), individual elements
can be repeated in the generated combinations:

Note that elements are treated as unique depending on their position
in the input, not their actual values.

The itertools.count() function now has a step argument that
allows incrementing by values other than 1. count() also
now allows keyword arguments, and using non-integer values such as
floats or Decimal instances. (Implemented by Raymond
Hettinger; issue 5032.)

Updated module: The json module was upgraded to version 2.0.9 of the
simplejson package, which includes a C extension that makes
encoding and decoding faster.
(Contributed by Bob Ippolito; issue 4136.)

To support the new collections.OrderedDict type, json.load()
now has an optional object_pairs_hook parameter that will be called
with any object literal that decodes to a list of pairs.
(Contributed by Raymond Hettinger; issue 5381.)

The mailbox module’s Maildir class now records the
timestamp on the directories it reads, and only re-reads them if the
modification time has subsequently changed. This improves
performance by avoiding unneeded directory scans. (Fixed by
A.M. Kuchling and Antoine Pitrou; issue 1607951, issue 6896.)

New functions: the math module gained
erf() and erfc() for the error function and the complementary error function,
expm1() which computes e**x-1 with more precision than
using exp() and subtracting 1,
gamma() for the Gamma function, and
lgamma() for the natural log of the Gamma function.
(Contributed by Mark Dickinson and nirinA raseliarison; issue 3366.)

The multiprocessing module’s Manager* classes
can now be passed a callable that will be called whenever
a subprocess is started, along with a set of arguments that will be
passed to the callable.
(Contributed by lekma; issue 5585.)

The Pool class, which controls a pool of worker processes,
now has an optional maxtasksperchild parameter. Worker processes
will perform the specified number of tasks and then exit, causing the
Pool to start a new worker. This is useful if tasks may leak
memory or other resources, or if some tasks will cause the worker to
become very large.
(Contributed by Charles Cazabon; issue 6963.)

New functions: the os module wraps the following POSIX system
calls: getresgid() and getresuid(), which return the
real, effective, and saved GIDs and UIDs;
setresgid() and setresuid(), which set
real, effective, and saved GIDs and UIDs to new values;
initgroups(), which initialize the group access list
for the current process. (GID/UID functions
contributed by Travis H.; issue 6508. Support for initgroups added
by Jean-Paul Calderone; issue 7333.)

The os.fork() function now re-initializes the import lock in
the child process; this fixes problems on Solaris when fork()
is called from a thread. (Fixed by Zsolt Cserna; issue 7242.)

The pydoc module now has help for the various symbols that Python
uses. You can now do help('<<') or help('@'), for example.
(Contributed by David Laban; issue 4739.)

The re module’s split(), sub(), and subn()
now accept an optional flags argument, for consistency with the
other functions in the module. (Added by Gregory P. Smith.)

New function: run_path() in the runpy module
will execute the code at a provided path argument. path can be
the path of a Python source file (example.py), a compiled
bytecode file (example.pyc), a directory
(./package/), or a zip archive (example.zip). If a
directory or zip path is provided, it will be added to the front of
sys.path and the module __main__ will be imported. It’s
expected that the directory or zip contains a __main__.py;
if it doesn’t, some other __main__.py might be imported from
a location later in sys.path. This makes more of the machinery
of runpy available to scripts that want to mimic the way
Python’s command line processes an explicit path name.
(Added by Nick Coghlan; issue 6816.)

New function: in the shutil module, make_archive()
takes a filename, archive type (zip or tar-format), and a directory
path, and creates an archive containing the directory’s contents.
(Added by Tarek Ziadé.)

shutil‘s copyfile() and copytree()
functions now raise a SpecialFileError exception when
asked to copy a named pipe. Previously the code would treat
named pipes like a regular file by opening them for reading, and
this would block indefinitely. (Fixed by Antoine Pitrou; issue 3002.)

The signal module no longer re-installs the signal handler
unless this is truly necessary, which fixes a bug that could make it
impossible to catch the EINTR signal robustly. (Fixed by
Charles-Francois Natali; issue 8354.)

New functions: in the site module, three new functions
return various site- and user-specific paths.
getsitepackages() returns a list containing all
global site-packages directories,
getusersitepackages() returns the path of the user’s
site-packages directory, and
getuserbase() returns the value of the USER_BASE
environment variable, giving the path to a directory that can be used
to store data.
(Contributed by Tarek Ziadé; issue 6693.)

The site module now reports exceptions occurring
when the sitecustomize module is imported, and will no longer
catch and swallow the KeyboardInterrupt exception. (Fixed by
Victor Stinner; issue 3137.)

The create_connection() function
gained a source_address parameter, a (host,port) 2-tuple
giving the source address that will be used for the connection.
(Contributed by Eldon Ziegler; issue 3972.)

The SocketServer module’s TCPServer class now
supports socket timeouts and disabling the Nagle algorithm.
The disable_nagle_algorithm class attribute
defaults to False; if overridden to be true,
new request connections will have the TCP_NODELAY option set to
prevent buffering many small sends into a single TCP packet.
The timeout class attribute can hold
a timeout in seconds that will be applied to the request socket; if
no request is received within that time, handle_timeout()
will be called and handle_request() will return.
(Contributed by Kristján Valur Jónsson; issue 6192 and issue 6267.)

Updated module: the sqlite3 module has been updated to
version 2.6.0 of the pysqlite package. Version 2.6.0 includes a number of bugfixes, and adds
the ability to load SQLite extensions from shared libraries.
Call the enable_load_extension(True) method to enable extensions,
and then call load_extension() to load a particular shared library.
(Updated by Gerhard Häring.)

The ssl module’s SSLSocket objects now support the
buffer API, which fixed a test suite failure (fix by Antoine Pitrou;
issue 7133) and automatically set
OpenSSL’s SSL_MODE_AUTO_RETRY, which will prevent an error
code being returned from recv() operations that trigger an SSL
renegotiation (fix by Antoine Pitrou; issue 8222).

Another change makes the extension load all of OpenSSL’s ciphers and
digest algorithms so that they’re all available. Some SSL
certificates couldn’t be verified, reporting an “unknown algorithm”
error. (Reported by Beda Kosata, and fixed by Antoine Pitrou;
issue 8484.)

The struct module will no longer silently ignore overflow
errors when a value is too large for a particular integer format
code (one of bBhHiIlLqQ); it now always raises a
struct.error exception. (Changed by Mark Dickinson;
issue 1523.) The pack() function will also
attempt to use __index__() to convert and pack non-integers
before trying the __int__() method or reporting an error.
(Changed by Mark Dickinson; issue 8300.)

New function: the subprocess module’s
check_output() runs a command with a specified set of arguments
and returns the command’s output as a string when the command runs without
error, or raises a CalledProcessError exception otherwise.

The subprocess module will now retry its internal system calls
on receiving an EINTR signal. (Reported by several people; final
patch by Gregory P. Smith in issue 1068268.)

New function: is_declared_global() in the symtable module
returns true for variables that are explicitly declared to be global,
false for ones that are implicitly global.
(Contributed by Jeremy Hylton.)

The syslog module will now use the value of sys.argv[0] as the
identifier instead of the previous default value of 'python'.
(Changed by Sean Reifschneider; issue 8451.)

The sys.version_info value is now a named tuple, with attributes
named major, minor, micro,
releaselevel, and serial. (Contributed by Ross
Light; issue 4285.)

The tarfile module’s default error handling has changed, to
no longer suppress fatal errors. The default error level was previously 0,
which meant that errors would only result in a message being written to the
debug log, but because the debug log is not activated by default,
these errors go unnoticed. The default error level is now 1,
which raises an exception if there’s an error.
(Changed by Lars Gustäbel; issue 7357.)

tarfile now supports filtering the TarInfo
objects being added to a tar file. When you call add(),
you may supply an optional filter argument
that’s a callable. The filter callable will be passed the
TarInfo for every file being added, and can modify and return it.
If the callable returns None, the file will be excluded from the
resulting archive. This is more powerful than the existing
exclude argument, which has therefore been deprecated.
(Added by Lars Gustäbel; issue 6856.)
The TarFile class also now supports the context management protocol.
(Added by Lars Gustäbel; issue 7232.)

The wait() method of the threading.Event class
now returns the internal flag on exit. This means the method will usually
return true because wait() is supposed to block until the
internal flag becomes true. The return value will only be false if
a timeout was provided and the operation timed out.
(Contributed by Tim Lesher; issue 1674032.)

The Unicode database provided by the unicodedata module is
now used internally to determine which characters are numeric,
whitespace, or represent line breaks. The database also
includes information from the Unihan.txt data file (patch
by Anders Chrigström and Amaury Forgeot d’Arc; issue 1571184)
and has been updated to version 5.2.0 (updated by
Florent Xicluna; issue 8024).

The urlparse module’s urlsplit() now handles
unknown URL schemes in a fashion compliant with RFC 3986: if the
URL is of the form "<something>://...", the text before the
:// is treated as the scheme, even if it’s a made-up scheme that
the module doesn’t know about. This change may break code that
worked around the old behaviour. For example, Python 2.6.4 or 2.5
will return the following:

New class: the WeakSet class in the weakref
module is a set that only holds weak references to its elements; elements
will be removed once there are no references pointing to them.
(Originally implemented in Python 3.x by Raymond Hettinger, and backported
to 2.7 by Michael Foord.)

The XML-RPC client and server, provided by the xmlrpclib and
SimpleXMLRPCServer modules, have improved performance by
supporting HTTP/1.1 keep-alive and by optionally using gzip encoding
to compress the XML being exchanged. The gzip compression is
controlled by the encode_threshold attribute of
SimpleXMLRPCRequestHandler, which contains a size in bytes;
responses larger than this will be compressed.
(Contributed by Kristján Valur Jónsson; issue 6267.)

The zipfile module’s ZipFile now supports the context
management protocol, so you can write withzipfile.ZipFile(...)asf:.
(Contributed by Brian Curtin; issue 5511.)

zipfile now also supports archiving empty directories and
extracts them correctly. (Fixed by Kuba Wieczorek; issue 4710.)
Reading files out of an archive is faster, and interleaving
read() and readline() now works correctly.
(Contributed by Nir Aides; issue 7610.)

The is_zipfile() function now
accepts a file object, in addition to the path names accepted in earlier
versions. (Contributed by Gabriel Genellina; issue 4756.)

The writestr() method now has an optional compress_type parameter
that lets you override the default compression method specified in the
ZipFile constructor. (Contributed by Ronald Oussoren;
issue 6003.)

Python 3.1 includes the importlib package, a re-implementation
of the logic underlying Python’s import statement.
importlib is useful for implementors of Python interpreters and
to users who wish to write new importers that can participate in the
import process. Python 2.7 doesn’t contain the complete
importlib package, but instead has a tiny subset that contains
a single function, import_module().

import_module(name,package=None) imports a module. name is
a string containing the module or package’s name. It’s possible to do
relative imports by providing a string that begins with a .
character, such as ..utils.errors. For relative imports, the
package argument must be provided and is the name of the package that
will be used as the anchor for
the relative import. import_module() both inserts the imported
module into sys.modules and returns the module object.

The sysconfig module has been pulled out of the Distutils
package, becoming a new top-level module in its own right.
sysconfig provides functions for getting information about
Python’s build process: compiler switches, installation paths, the
platform name, and whether Python is running from its source
directory.

Some of the functions in the module are:

get_config_var() returns variables from Python’s
Makefile and the pyconfig.h file.

get_config_vars() returns a dictionary containing
all of the configuration variables.

is_python_build() returns true if you’re running a
binary from a Python source tree, and false otherwise.

Consult the sysconfig documentation for more details and for
a complete list of functions.

The Distutils package and sysconfig are now maintained by Tarek
Ziadé, who has also started a Distutils2 package (source repository at
https://hg.python.org/distutils2/) for developing a next-generation
version of Distutils.

Tcl/Tk 8.5 includes a set of themed widgets that re-implement basic Tk
widgets but have a more customizable appearance and can therefore more
closely resemble the native platform’s widgets. This widget
set was originally called Tile, but was renamed to Ttk (for “themed Tk”)
on being added to Tcl/Tck release 8.5.

The ttk module was written by Guilherme Polo and added in
issue 2983. An alternate version called Tile.py, written by
Martin Franklin and maintained by Kevin Walzer, was proposed for
inclusion in issue 2618, but the authors argued that Guilherme
Polo’s work was more comprehensive.

The unittest module was greatly enhanced; many
new features were added. Most of these features were implemented
by Michael Foord, unless otherwise noted. The enhanced version of
the module is downloadable separately for use with Python versions 2.4 to 2.6,
packaged as the unittest2 package, from
https://pypi.python.org/pypi/unittest2.

When used from the command line, the module can automatically discover
tests. It’s not as fancy as py.test or
nose, but provides a simple way
to run tests kept within a set of package directories. For example,
the following command will search the test/ subdirectory for
any importable test files named test*.py:

-b or --buffer will buffer the standard output
and standard error streams during each test. If the test passes,
any resulting output will be discarded; on failure, the buffered
output will be displayed.

-c or --catch will cause the control-C interrupt
to be handled more gracefully. Instead of interrupting the test
process immediately, the currently running test will be completed
and then the partial results up to the interruption will be reported.
If you’re impatient, a second press of control-C will cause an immediate
interruption.

This control-C handler tries to avoid causing problems when the code
being tested or the tests being run have defined a signal handler of
their own, by noticing that a signal handler was already set and
calling it. If this doesn’t work for you, there’s a
removeHandler() decorator that can be used to mark tests that
should have the control-C handling disabled.

-f or --failfast makes
test execution stop immediately when a test fails instead of
continuing to execute further tests. (Suggested by Cliff Dyer and
implemented by Michael Foord; issue 8074.)

The progress messages now show ‘x’ for expected failures
and ‘u’ for unexpected successes when run in verbose mode.
(Contributed by Benjamin Peterson.)

Module- and class-level setup and teardown fixtures are now supported.
Modules can contain setUpModule() and tearDownModule()
functions. Classes can have setUpClass() and
tearDownClass() methods that must be defined as class methods
(using @classmethod or equivalent). These functions and
methods are invoked when the test runner switches to a test case in a
different module or class.

A number of new methods were added that provide more specialized
tests. Many of these methods were written by Google engineers
for use in their test suites; Gregory P. Smith, Michael Foord, and
GvR worked on merging them into Python’s version of unittest.

assertMultiLineEqual() compares two strings, and if they’re
not equal, displays a helpful comparison that highlights the
differences in the two strings. This comparison is now used by
default when Unicode strings are compared with assertEqual().

assertRegexpMatches() and
assertNotRegexpMatches() checks whether the
first argument is a string matching or not matching the regular
expression provided as the second argument (issue 8038).

assertRaisesRegexp() checks whether a particular exception
is raised, and then also checks that the string representation of
the exception matches the provided regular expression.

assertItemsEqual() tests whether two provided sequences
contain the same elements.

assertSetEqual() compares whether two sets are equal, and
only reports the differences between the sets in case of error.

Similarly, assertListEqual() and assertTupleEqual()
compare the specified types and explain any differences without necessarily
printing their full values; these methods are now used by default
when comparing lists and tuples using assertEqual().
More generally, assertSequenceEqual() compares two sequences
and can optionally check whether both sequences are of a
particular type.

assertDictEqual() compares two dictionaries and reports the
differences; it’s now used by default when you compare two dictionaries
using assertEqual(). assertDictContainsSubset() checks whether
all of the key/value pairs in first are found in second.

assertAlmostEqual() and assertNotAlmostEqual() test
whether first and second are approximately equal. This method
can either round their difference to an optionally-specified number
of places (the default is 7) and compare it to zero, or require
the difference to be smaller than a supplied delta value.

A new hook lets you extend the assertEqual() method to handle
new data types. The addTypeEqualityFunc() method takes a type
object and a function. The function will be used when both of the
objects being compared are of the specified type. This function
should compare the two objects and raise an exception if they don’t
match; it’s a good idea for the function to provide additional
information about why the two objects aren’t matching, much as the new
sequence comparison methods do.

With all these changes, the unittest.py was becoming awkwardly
large, so the module was turned into a package and the code split into
several files (by Benjamin Peterson). This doesn’t affect how the
module is imported or used.

The version of the ElementTree library included with Python was updated to
version 1.3. Some of the new features are:

The various parsing functions now take a parser keyword argument
giving an XMLParser instance that will
be used. This makes it possible to override the file’s internal encoding:

p=ET.XMLParser(encoding='utf-8')t=ET.XML("""<root/>""",parser=p)

Errors in parsing XML now raise a ParseError exception, whose
instances have a position attribute
containing a (line, column) tuple giving the location of the problem.

ElementTree’s code for converting trees to a string has been
significantly reworked, making it roughly twice as fast in many
cases. The ElementTree.write()
and Element.write() methods now have a method parameter that can be
“xml” (the default), “html”, or “text”. HTML mode will output empty
elements as <empty></empty> instead of <empty/>, and text
mode will skip over elements and only output the text chunks. If
you set the tag attribute of an element to None but
leave its children in place, the element will be omitted when the
tree is written out, so you don’t need to do more extensive rearrangement
to remove a single element.

Namespace handling has also been improved. All xmlns:<whatever>
declarations are now output on the root element, not scattered throughout
the resulting XML. You can set the default namespace for a tree
by setting the default_namespace attribute and can
register new prefixes with register_namespace(). In XML mode,
you can use the true/false xml_declaration parameter to suppress the
XML declaration.

New Element method:
extend() appends the items from a
sequence to the element’s children. Elements themselves behave like
sequences, so it’s easy to move children from one element to
another:

New Element method:
iter() yields the children of the
element as a generator. It’s also possible to write forchildinelem: to loop over an element’s children. The existing method
getiterator() is now deprecated, as is getchildren()
which constructs and returns a list of children.

New Element method:
itertext() yields all chunks of
text that are descendants of the element. For example:

Deprecated: using an element as a Boolean (i.e., ifelem:) would
return true if the element had any children, or false if there were
no children. This behaviour is confusing – None is false, but
so is a childless element? – so it will now trigger a
FutureWarning. In your code, you should be explicit: write
len(elem)!=0 if you’re interested in the number of children,
or elemisnotNone.

The latest release of the GNU Debugger, GDB 7, can be scripted
using Python.
When you begin debugging an executable program P, GDB will look for
a file named P-gdb.py and automatically read it. Dave Malcolm
contributed a python-gdb.py that adds a number of
commands useful when debugging Python itself. For example,
py-up and py-down go up or down one Python stack frame,
which usually corresponds to several C stack frames. py-print
prints the value of a Python variable, and py-bt prints the
Python stack trace. (Added as a result of issue 8032.)

If you use the .gdbinit file provided with Python,
the “pyo” macro in the 2.7 version now works correctly when the thread being
debugged doesn’t hold the GIL; the macro now acquires it before printing.
(Contributed by Victor Stinner; issue 3632.)

Py_AddPendingCall() is now thread-safe, letting any
worker thread submit notifications to the main Python thread. This
is particularly useful for asynchronous IO operations.
(Contributed by Kristján Valur Jónsson; issue 4293.)

New function: PyCode_NewEmpty() creates an empty code object;
only the filename, function name, and first line number are required.
This is useful for extension modules that are attempting to
construct a more useful traceback stack. Previously such
extensions needed to call PyCode_New(), which had many
more arguments. (Added by Jeffrey Yasskin.)

New function: PyFrame_GetLineNumber() takes a frame object
and returns the line number that the frame is currently executing.
Previously code would need to get the index of the bytecode
instruction currently executing, and then look up the line number
corresponding to that address. (Added by Jeffrey Yasskin.)

New function: stemming from the rewrite of string-to-float conversion,
a new PyOS_string_to_double() function was added. The old
PyOS_ascii_strtod() and PyOS_ascii_atof() functions
are now deprecated.

New function: PySys_SetArgvEx() sets the value of
sys.argv and can optionally update sys.path to include the
directory containing the script named by sys.argv[0] depending
on the value of an updatepath parameter.

This function was added to close a security hole for applications
that embed Python. The old function, PySys_SetArgv(), would
always update sys.path, and sometimes it would add the current
directory. This meant that, if you ran an application embedding
Python in a directory controlled by someone else, attackers could
put a Trojan-horse module in the directory (say, a file named
os.py) that your application would then import and run.

If you maintain a C/C++ application that embeds Python, check
whether you’re calling PySys_SetArgv() and carefully consider
whether the application should be using PySys_SetArgvEx()
with updatepath set to false.

New macros: the Python header files now define the following macros:
Py_ISALNUM,
Py_ISALPHA,
Py_ISDIGIT,
Py_ISLOWER,
Py_ISSPACE,
Py_ISUPPER,
Py_ISXDIGIT,
Py_TOLOWER, and Py_TOUPPER.
All of these functions are analogous to the C
standard macros for classifying characters, but ignore the current
locale setting, because in
several places Python needs to analyze characters in a
locale-independent way. (Added by Eric Smith;
issue 5793.)

Removed function: PyEval_CallObject is now only available
as a macro. A function version was being kept around to preserve
ABI linking compatibility, but that was in 1997; it can certainly be
deleted by now. (Removed by Antoine Pitrou; issue 8276.)

The complicated interaction between threads and process forking has
been changed. Previously, the child process created by
os.fork() might fail because the child is created with only a
single thread running, the thread performing the os.fork().
If other threads were holding a lock, such as Python’s import lock,
when the fork was performed, the lock would still be marked as
“held” in the new process. But in the child process nothing would
ever release the lock, since the other threads weren’t replicated,
and the child process would no longer be able to perform imports.

Python 2.7 acquires the import lock before performing an
os.fork(), and will also clean up any locks created using the
threading module. C extension modules that have internal
locks, or that call fork() themselves, will not benefit
from this clean-up.

The Py_Finalize() function now calls the internal
threading._shutdown() function; this prevents some exceptions from
being raised when an interpreter shuts down.
(Patch by Adam Olsen; issue 1722344.)

When using the PyMemberDef structure to define attributes
of a type, Python will no longer let you try to delete or set a
T_STRING_INPLACE attribute.

Global symbols defined by the ctypes module are now prefixed
with Py, or with _ctypes. (Implemented by Thomas
Heller; issue 3102.)

New configure option: the --with-system-expat switch allows
building the pyexpat module to use the system Expat library.
(Contributed by Arfrever Frehtes Taifersar Arahesis; issue 7609.)

New configure option: the
--with-valgrind option will now disable the pymalloc
allocator, which is difficult for the Valgrind memory-error detector
to analyze correctly.
Valgrind will therefore be better at detecting memory leaks and
overruns. (Contributed by James Henstridge; issue 2422.)

New configure option: you can now supply an empty string to
--with-dbmliborder= in order to disable all of the various
DBM modules. (Added by Arfrever Frehtes Taifersar Arahesis;
issue 6491.)

The configure script now checks for floating-point rounding bugs
on certain 32-bit Intel chips and defines a X87_DOUBLE_ROUNDING
preprocessor definition. No code currently uses this definition,
but it’s available if anyone wishes to use it.
(Added by Mark Dickinson; issue 2937.)

Python 3.1 adds a new C datatype, PyCapsule, for providing a
C API to an extension module. A capsule is essentially the holder of
a C void* pointer, and is made available as a module attribute; for
example, the socket module’s API is exposed as socket.CAPI,
and unicodedata exposes ucnhash_CAPI. Other extensions
can import the module, access its dictionary to get the capsule
object, and then get the void* pointer, which will usually point
to an array of pointers to the module’s various API functions.

There is an existing data type already used for this,
PyCObject, but it doesn’t provide type safety. Evil code
written in pure Python could cause a segmentation fault by taking a
PyCObject from module A and somehow substituting it for the
PyCObject in module B. Capsules know their own name,
and getting the pointer requires providing the name:

Python 2.7 now uses capsules internally to provide various
extension-module APIs, but the PyCObject_AsVoidPtr() was
modified to handle capsules, preserving compile-time compatibility
with the CObject interface. Use of
PyCObject_AsVoidPtr() will signal a
PendingDeprecationWarning, which is silent by default.

Implemented in Python 3.1 and backported to 2.7 by Larry Hastings;
discussed in issue 5630.

The msvcrt module now contains some constants from
the crtassem.h header file:
CRT_ASSEMBLY_VERSION,
VC_ASSEMBLY_PUBLICKEYTOKEN,
and LIBRARIES_ASSEMBLY_NAME_PREFIX.
(Contributed by David Cournapeau; issue 4365.)

The _winreg module for accessing the registry now implements
the CreateKeyEx() and DeleteKeyEx()
functions, extended versions of previously-supported functions that
take several extra arguments. The DisableReflectionKey(),
EnableReflectionKey(), and QueryReflectionKey()
were also tested and documented.
(Implemented by Brian Curtin: issue 7347.)

The new _beginthreadex() API is used to start threads, and
the native thread-local storage functions are now used.
(Contributed by Kristján Valur Jónsson; issue 3582.)

The os.kill() function now works on Windows. The signal value
can be the constants CTRL_C_EVENT,
CTRL_BREAK_EVENT, or any integer. The first two constants
will send Control-C and Control-Break keystroke events to
subprocesses; any other value will use the TerminateProcess()
API. (Contributed by Miki Tebeka; issue 1220212.)

The path /Library/Python/2.7/site-packages is now appended to
sys.path, in order to share added packages between the system
installation and a user-installed copy of the same version.
(Changed by Ronald Oussoren; issue 4865.)

Two benchmark scripts, iobench and ccbench, were
added to the Tools directory. iobench measures the
speed of the built-in file I/O objects returned by open()
while performing various operations, and ccbench is a
concurrency benchmark that tries to measure computing throughput,
thread switching latency, and IO processing bandwidth when
performing several tasks using a varying number of threads.

When importing a module from a .pyc or .pyo file
with an existing .py counterpart, the co_filename
attributes of the resulting code objects are overwritten when the
original filename is obsolete. This can happen if the file has been
renamed, moved, or is accessed through different paths. (Patch by
Ziga Seilnacht and Jean-Paul Calderone; issue 1180193.)

The regrtest.py script now takes a --randseed=
switch that takes an integer that will be used as the random seed
for the -r option that executes tests in random order.
The -r option also reports the seed that was used
(Added by Collin Winter.)

Another regrtest.py switch is -j, which
takes an integer specifying how many tests run in parallel. This
allows reducing the total runtime on multi-core machines.
This option is compatible with several other options, including the
-R switch which is known to produce long runtimes.
(Added by Antoine Pitrou, issue 6152.) This can also be used
with a new -F switch that runs selected tests in a loop
until they fail. (Added by Antoine Pitrou; issue 7312.)

When executed as a script, the py_compile.py module now
accepts '-' as an argument, which will read standard input for
the list of filenames to be compiled. (Contributed by Piotr
Ożarowski; issue 8233.)

This section lists previously described changes and other bugfixes
that may require changes to your code:

The range() function processes its arguments more
consistently; it will now call __int__() on non-float,
non-integer arguments that are supplied to it. (Fixed by Alexander
Belopolsky; issue 1533.)

The string format() method changed the default precision used
for floating-point and complex numbers from 6 decimal
places to 12, which matches the precision used by str().
(Changed by Eric Smith; issue 5920.)

Because of an optimization for the with statement, the special
methods __enter__() and __exit__() must belong to the object’s
type, and cannot be directly attached to the object’s instance. This
affects new-style classes (derived from object) and C extension
types. (issue 6101.)

Due to a bug in Python 2.6, the exc_value parameter to
__exit__() methods was often the string representation of the
exception, not an instance. This was fixed in 2.7, so exc_value
will be an instance as expected. (Fixed by Florent Xicluna;
issue 7853.)

When a restricted set of attributes were set using __slots__,
deleting an unset attribute would not raise AttributeError
as you would expect. Fixed by Benjamin Peterson; issue 7604.)

In the standard library:

Operations with datetime instances that resulted in a year
falling outside the supported range didn’t always raise
OverflowError. Such errors are now checked more carefully
and will now raise the exception. (Reported by Mark Leander, patch
by Anand B. Pillai and Alexander Belopolsky; issue 7150.)

When using Decimal instances with a string’s
format() method, the default alignment was previously
left-alignment. This has been changed to right-alignment, which might
change the output of your programs.
(Changed by Mark Dickinson; issue 6857.)

The readline() method of StringIO objects now does
nothing when a negative length is requested, as other file-like
objects do. (issue 7348).

The syslog module will now use the value of sys.argv[0] as the
identifier instead of the previous default value of 'python'.
(Changed by Sean Reifschneider; issue 8451.)

The tarfile module’s default error handling has changed, to
no longer suppress fatal errors. The default error level was previously 0,
which meant that errors would only result in a message being written to the
debug log, but because the debug log is not activated by default,
these errors go unnoticed. The default error level is now 1,
which raises an exception if there’s an error.
(Changed by Lars Gustäbel; issue 7357.)

The urlparse module’s urlsplit() now handles
unknown URL schemes in a fashion compliant with RFC 3986: if the
URL is of the form "<something>://...", the text before the
:// is treated as the scheme, even if it’s a made-up scheme that
the module doesn’t know about. This change may break code that
worked around the old behaviour. For example, Python 2.6.4 or 2.5
will return the following:

New features may be added to Python 2.7 maintenance releases when the
situation genuinely calls for it. Any such additions must go through
the Python Enhancement Proposal process, and make a compelling case for why
they can’t be adequately addressed by either adding the new feature solely to
Python 3, or else by publishing it on the Python Package Index.

In addition to the specific proposals listed below, there is a general
exemption allowing new -3 warnings to be added in any Python 2.7
maintenance release.

PEP 434 describes a general exemption for changes made to the IDLE
development environment shipped along with Python. This exemption makes it
possible for the IDLE developers to provide a more consistent user
experience across all supported versions of Python 2 and 3.

For details of any IDLE changes, refer to the NEWS file for the specific
release.

PEP 466 describes a number of network security enhancement proposals
that have been approved for inclusion in Python 2.7 maintenance releases,
with the first of those changes appearing in the Python 2.7.7 release.

hashlib.pbkdf2_hmac() was backported from Python 3 to make a hashing
algorithm suitable for secure password storage broadly available to Python
2 applications. (Contributed by Alex Gaynor; issue 21304.)

OpenSSL 1.0.1h was upgraded for the official Windows installers published on
python.org. (contributed by Zachary Ware in issue 21671 for CVE-2014-0224)

Most of Python 3.4’s ssl module was backported. This means ssl
now supports Server Name Indication, TLS1.x settings, access to the platform
certificate store, the SSLContext class, and other
features. (Contributed by Alex Gaynor and David Reid; issue 21308.)

os.urandom() was changed to cache a file descriptor to /dev/urandom
instead of reopening /dev/urandom on every call. (Contributed by Alex
Gaynor; issue 21305.)

The author would like to thank the following people for offering
suggestions, corrections and assistance with various drafts of this
article: Nick Coghlan, Philip Jenvey, Ryan Lovett, R. David Murray,
Hugh Secker-Walker.